1. Field of the Invention
The present invention relates to a continuous process for reducing the concentration of water soluble inorganic and organic selenium species in aqueous solutions. More particularly, the process relates to the removal of selenium from a variety of refinery process water and wastewater streams such as stripped sour water, phenolic stripped sour water, reverse osmosis reject water from the pretreatment of refinery wastewater or stripped sour water, and ion exchange regeneration wastewater from the pretreatment of refinery wastewater or stripped sour water. The invention provides for the recovery of selenium from these aqueous process or wastewater streams to allow refineries to meet increasingly stringent regulatory requirements for the concentration and mass emission rates of selenium in discharged refinery wastewater.
2. Background
Selenium compounds are present in trace quantities in petroleum as high molecular weight, organometallic molecules. The concentration of selenium in petroleum varies by production location and ranges from non-detect to over 1.0 part per million (ppm) by weight. The organic selenium compounds in the petroleum are distilled into the heavier hydrocarbon fractions during refining. The selenium is liberated from the heavy hydrocarbons by catalytic or thermal cracking processes, such as hydrocrackers and delayed cokers, removed as inorganic compounds and concentrated in the overhead product and vapor streams. These inorganic selenium compounds, like similar inorganic sulfur and nitrogen compounds such as hydrogen sulfide and ammonia, are very soluble in water and are generally removed from the overhead streams by contacting them with water. The sulfur and nitrogen species are typically volatile and can be removed from the "sour water" stream by stripping (contacting the water stream with a countercurrent stream of steam); but the selenium species are somewhat less volatile, and most of the selenium remains in the sour water even after stripping. Because high concentrations of selenium are toxic, it may be necessary for some refineries to reduce the concentration of selenium in the stripped sour water and other waste waters before they can be discharged to the environment.
There are a number of known methods of removing selenium from aqueous and hydrocarbon streams. These fall broadly into three categories: methods in which the selenium compounds are altered by chemical reaction, methods that rely on adsorption and methods that rely on membrane separation. Several of the reaction methods involve treatment with hydrogen or other reducing agents to convert the dissolved selenium species to elemental selenium. Other methods employ bacteria which accomplish the same reaction metabolically. Still other reaction methods, directed specifically toward the removal of hydrogen selenide, involve oxidation of the selenide to elemental selenium. A number of additional reaction methods are based on the reaction of selenium with various sulfur-containing compounds to form insoluble sulfides. Still other methods are known which employ electrolysis or anion exchange to extract selenium-containing ions or to replace them with less toxic anions.
A number of methods are also known for removing selenium from waste water by adsorbing the selenium compounds on a solid adsorbent. Some of the materials that have been employed as adsorbents include zeolites, dithiocarbamates and their polymers, and metal salts such as ferric chloride and ferric sulfate. Most of these adsorption methods are effective where the selenium is in the form of selenite anions, but not effective in removing the selenocyanate ion, SeCN.sup.-, which is the predominant form of selenium in refinery waste water and stripped sour water. Attempts have been made to oxidize the selenocyanate prior to introducing the adsorbent; oxidizing agents which have been employed have included air, ozone, hydrogen peroxide, and chlorine dioxide. None of these oxidants has proven entirely satisfactory: air is ineffective for oxidizing selenocyanate, while ozone and peroxide require highly alkaline conditions for maximum effectiveness. Chlorine dioxide oxidizes selenocyanate effectively at neutral pH, but it is unstable in aqueous solution and would have to be generated as needed.
U.S. Pat. No. 5,200,082 discloses a selenium removal method in which the selenium compounds in the water are first reduced to elemental selenium, then re-oxidized to selenite and removed by a conventional adsorption method as described above. Because the process requires the reintroduction of sulfur into the water, it is unsuitable for water streams such as stripped sour water which have previously undergone sulfur removal.